The present invention generally relates to forced air dryers of the batch type, and more particularly to methods and apparatus for controlling air recirculation during the drying cycle to optimize the fuel efficiency of such dryers.
It is a well known technique in commercial fabric dryers to elevate the temperature of the moisture-laden air removed from the drying chamber and recirculate it back into the chamber. U.S. Pat. No. 4,268,247 titled "Method for Drying Fabrics" discusses such a method wherein the reuse of a given amount of moisture-laden hot air, mixed with a predetermined amount of fresh air, results in a reduction of fuel consumption as compared to a system which only heats fresh air for introduction into the drying chamber.
It is discussed in the noted patent, as well as in my U.S. Pat. No. 4,267,643, that the temperature of hot air removed from the drying chamber may be used as an indication of the progress of the drying cycle to control the dryer heater.
While the drying systems known in the art achieve an efficiency of operation by recirculating a predetermined portion of the air removed from the drying chamber, the drawback of such systems is that during the early part of the drying cycle the humidity of the air removed from the chamber is high, and thus the reuse of such air reduces the effectiveness of the dryer to further evaporate moisture from the fabrics. On the other hand, in the latter part of the drying cycle, the air removed contains less evaporated moisture and thus a higher percentage of the removed air should be reintroduced into the dryer.
In textile dryers it is known that one can measure the humidity of hot air removed from the drying chamber to control the exhaust damper to regulate the amount of hot air which is exhausted from the system.
While the moisture content of the hot air removed from the drying chamber can be measured over the period of the drying cycle, the value thereof changes as a function of a host of variables, such as for example; the temperature and humidity of the fresh air added, the amount and type of fabrics to be dryed and the temperature to which the reused air is heated. It is evident that there is a need to control the amount of hot air reused based upon the moisture content of such air at various periods of time in the drying cycle. The amount of moist hot air exhausted from the drying system should be correspondingly replaced by cooler dry air which is then heated and recirculated. In other words, during the early part of the drying cycle, little hot air should be reused, and large amounts of fresh air should be drawn into the system. As the drying cycle progresses, a correspondingly larger amount of hot air in the dryer should be reused, and less fresh air introduced into the system. It is a corollary to the foregoing that as the amount of dry hot air reused increases, less heat energy needs to be added to the system. It is further evident that the portion of hot, wet air which is exhausted must be replaced within fresh air, prefereably from outdoors in order to obtain air with the lowest possible moisture content.
In order to optimize the efficiency of drying systems parameters, other than air temperatures within the drying system, such as the temperature of the fresh air, should be monitored and used as indications when changes in the drying cycle are necessary. Because of the large numbers of conventional dryers presently in existence, there is a need to provide an adjunct to thereby modernize such systems without extensive modification or reworking of the established system.